Conventionally, various types of anti-counterfeiting technology are used for articles requiring authentication and counterfeit prevention, such as cards (cash card, credit card, check card, etc.), gold notes, identifications, and important documents. For example, as the anti-counterfeiting technology, special inks such as optically variable ink and fluorescence ink, a hologram and the like are available. In addition to the anti-counterfeit technology, an authenticity determination method for detection and authentication of the anti-counterfeiting technology is used to perform authenticity determination using the anti-counterfeiting technology. To perform the authenticity determination more conveniently, an authenticity determination device using the authenticity determination method is used.
The hologram used in the anti-counterfeiting technology is a medium obtained by recording wavefront information of an object light on a photosensitive material as interference fringes by making two coherent lights (object light and reference light) interfere with each other. When the hologram is illuminated with a light close to a reference light at the time of interference fringes recording, a diffraction phenomenon due to the interference fringes occurs, whereby a wavefront close to the original object light can be reproduced. Such a hologram is classified into several types (relief hologram, volume hologram, etc.) depending on a recording form of the interference fringes.
The relief hologram (relief-type hologram) is a hologram obtained by shaping interference fringes generated by interference of light as a fine uneven pattern on a hologram layer surface. The volume hologram (volume type hologram) is a hologram obtained by three-dimensionally recording interference fringes generated by interference of light in the thickness direction as fringes having different refractive indices or transmittances inside a hologram layer.
In the volume hologram, the interference fringes are three-dimensionally recorded in the thickness direction of a recording material, so that a light satisfying a condition on a waveform and a light path direction, which is called “Bragg condition” is diffracted with high diffraction efficiency. That is, the volume hologram has wavelength selectivity and angle selectivity that diffract a specific wavelength at a specific angle. The volume hologram can be roughly classified into a reflective type and a transmissive type depending on the diffraction direction of light. The reflective volume hologram has stronger wavelength selectivity than that of the transmissive volume hologram, so that the wavelength of a reflected/diffracted light is limited to a narrow range (specific color).
Conventionally, as the technology to detect and authenticate the volume hologram, there is disclosed a technology that irradiates a volume hologram laminate with an inspection light having a wavelength different from a specific wavelength of a volume hologram layer or with an inspection light having an angle different from a specific angle of the volume hologram layer so as to stably detect defects of the volume hologram laminate with high accuracy (see Patent Literature 1).